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# Modeling Rod End Ball Linkages

## Modeling Rod End Ball Linkages

(OP)
I'm attempting to model a statically load assembly in which the the assembly is held together by Spherical Rod End Ball Linkages.  The model consists of beam elements to model the two bodies fixed together with the Rod End Linkages (these are beam elements also with their associated properties).  To simulate the spherical rod end features, I'm releasing the end elements in all rotations.  When I attempt to run the model I get the dreaded Excessive Pivot Ratio Error.  I've check my model for being under constrained, and for the life of me I can not see where that might be the case.  I'm not real familiar with release elements, so I'm not sure if I'm using this feature correctly.  Thanks for any help and insight.

### RE: Modeling Rod End Ball Linkages

One suggestion would be to run an eigenvalueanalysis, frequencies. This might indicate if there is unwanted possible movement in your model.

Another way would be to remove the releases altogether and se what happens. Is there still a numerical problem?

But it can also be very simple: A rod with all rotations released in both ends are singular. It can rotate around its own axis. Release RX only in one end of the rod.

Good Luck

Thomas

### RE: Modeling Rod End Ball Linkages

When you pin the beam element ends, pin End A in the 456 directions and End B in the 56 directions or vice versa.
If you pin both ends of a beam element in torsion (direction 4) it is free to spin about the axial axis causing excessive pivot ratios. Pinning one end in torsion is equivalent to pinning both ends in torsion, since the beam can not transfer torsion when one end is pinned.

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